Mechanistic study on the role of design and management factors in regulating nitrogen treatment performance in constructed wetlands

Abstract

Surface flow constructed wetlands (SFCWs) are increasingly recognized as an effective solution for nitrogen in agricultural drainage, with their performance influenced by design and management. However, the mechanisms by which these factors regulate nitrogen removal remain unclear. Understanding these mechanisms is crucial to promoting and applying SFCWs. In this study, field pilot-scale SFCWs were subjected to orthogonal tests to investigate the mechanistic influence of aspect ratios (the ratio of length to width) (2:1, 4:1, 8:1), plant species (Typha latifolia, Juncus effusus, Vallisneria natans), and hydraulic loading rates (HLR) (0.23, 0.45, 0.68 m d⁻¹) on nitrogen removal. The results showed that N₂ emission (21.4% ∼ 67.4%) was the primary pathway for total nitrogen (TN) removal, followed by plant uptake (0.9% ∼ 58.6%), sediment storage (8.8% ∼ 44.0%), periphyton assimilation (2.9% ∼ 7.8%), NH₃ volatilization (1.9% ∼ 6.4%), and N₂O emission (0.4% ∼ 1.1%). Aspect ratio and HLR had weak effects on nitrogen treatment performance and transformations. In contrast, plant species significantly affected TN removal, nitrate nitrogen (${\mathrm{NO}}_3^{-}$-N) removal, NH₃ volatilization, and N₂ emission. The highest reaction rates for TN removal, ${\mathrm{NO}}_3^{-}$-N removal, and N₂ emission were observed under Vallisneria. However, NH₃ volatilization peaked under Juncus. Water pH and plant biomass could explain the impact of plant species on N₂ emission and NH₃ volatilization. Overall, plant species selection is more crucial than aspect ratio and HLR, with Vallisneria as the preferred plant species for efficient TN removal.